Remote communication using wireless equipment may rely on radio frequency (RF) technology. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles. Other applications of RF technology may include communication of data collected from remote sensors.
RF identification (RFID) tag systems have been developed to facilitate monitoring of remote objects and communication of data collected from remote sensors. As shown in FIG. 1, a basic RF tag system 10 may include three components, an antenna 12, a transceiver with decoder 14, and a transponder (commonly called an RFID tag) 16. In operation, the antenna 12 may emit electromagnetic radio signals generated by the transceiver 14 to activate the RFID tag 16. When the RFID tag 16 is activated, data can be read from or written to the RFID tag 16.
In some applications, the antenna 12 may be a component of the transceiver and decoder 14 to become an interrogator (or reader) 18. The reader 18 may activate or “wake up” the RFID tag 16 by radiating energy to the tag in an on/off pattern encoded in some time varying manner. When an RFID tag 16 passes through the electromagnetic radio waves 20, the RFID tag 16 detects the signal 20 and is activated. An example of one manner commonly used to activate an RFID tag is biphase encoding. When the reader 18 is done talking to the RFID tag 16, the reader 18 may then go to a continuous broadcast of energy that the RFID tag 16 uses to communicate information to the reader via backscatter methodologies. Data encoded in the RFID tag 16 may be communicated to the reader 18 by a data signal 22 through an antenna 23. The RFID tag 16 may modulate its antenna and put a subcarrier on the reader's backscattered carrier signal that could later be stripped off and demodulated. The subcarrier may use a time varying amplitude shifting modulation technique such as biphase modulation to encode the data into the subcarrier signal.
RFID tag communication systems may include systems where the RFID tags return data at a specific frequency associated with each RFID tag. For example, a reader may transmit a signal at one frequency, and each RFID tag can modulate the amplitude of its signal at a frequency separate from the frequency of any other RFID tag in the system. Such systems can allow the reader to simultaneously differentiate information received from multiple RFID tags. Further, the RFID tags may be configured to allow a tag to communicate at one of several frequencies and to adaptively avoid interference with other tags that may be communicating on an identical frequency. While an RFID tag may adaptively change the frequency at which it is communicating, the means for communicating information still relies on a method of modulating the amplitude of a signal in some time varying fashion to encode data in the signal.
Several disadvantages may be associated with encoding a signal from an RFID tag by modulating the amplitude of a signal in some time varying fashion. For a reader to decode a time varying amplitude modulated system, it may be required to accurately identify the amplitude change times. To accurately identify an amplitude change time, a minimum signal to noise threshold may need to be maintained to clearly detect the edge of an amplitude change. A reader may be required to sample the returned signal at some rate higher than twice the data bandwidth such as 10 times the data bandwidth. The reader may obtain little processing gain from averaging the signal sampled since signal averaging can smear the edges of amplitude changes and can raise the sample requirements further.
Thus, it would therefore be desirable to have a system and method for communicating between one or more RF tags and a reader such that the data transfer rate may be increased, and the error rate may decrease. It would be desirable to increase the data transfer rate and decrease the error rate without significantly raising the cost of a system. Further, it would be desirable to have a system and method for communicating between one or more RF tags and reader such that multiple RF tags can communicate with the reader simultaneously.